<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>盒子模型</title>
    <style> 
        #app{
            /* 默认状态是内容盒子 */
            /* box-sizing: content-box; */
            /* 修改为边框盒子 */
            box-sizing: border-box;
            width: 100px;
            height: 100px;
            border: 1px solid red;
            padding: 10px;
            padding-top: 20px;
            padding-left: 30px;
            margin: 10px;
            margin-bottom: 40px;
            margin-right: 80px;
            margin: 10px 20px 30px 40px;
        }
        </style>
</head>
<body>
    <div id="app">盒子模型</div>
        
    
</body>
</html>